1. Field of the Invention
This invention relates generally to a composite paper, and more particularly to a plastic-coated composite paper having improved printability.
2. Statement of the Prior Art
Paper is widely used in a variety of commercially-important applications, primarily due to its unique combination of desirable properties. For example, paper has traditionally been used as a substrate for printing because its excellent acceptance of ink provides for high resolution printing. Paper is also used in a variety of mechanical applications such as in the manufacture of flexible packaging (e.g., paper bags), and in the manufacture of corrugated and solid fiber boards for use in cartons. The physical properties of paper in such mechanical applications, including its stiffness, its tensile, tear and compression strength, its dead fold characteristic (i.e., the ability of paper to accept a fold and stay in position) and its relatively low cost on a cost-performance basis, are all particularly important. Furthermore, the printability of paper in mechanical applications is generally an important feature as it is often desirable or even necessary for information to be printed on paper products such as bags and boxes.
There are also certain other properties of conventional paper which have limited its use in many mechanical applications. For example, conventional paper readily absorbs moisture and has exceptionally poor wet strength. If conventional paper is exposed to even a moderate amount of moisture, it will absorb the moisture and deform or wrinkle. This property can be a serious shortcoming in the printing of books, manuals, maps, labels and the like since the printed product may ultimately be exposed to adversely moist conditions. That is, when conventional paper is exposed to relatively high levels of moisture, or is placed in direct contact with water, or exposed to the weather, it readily absorbs any ambient water and resultingly exhibits essentially no wet strength. For most paper products (e.g., paper bags, cartons, and the like), such exposure to moisture and resulting reduction in wet strength can destroy the product's usefulness.
A further problem that is encountered with conventional paper is its relatively poor property as a vapor barrier. As a result of this property, air, water vapor, and other solvent vapors can readily penetrate through the paper making it highly undesirable for use in bags or the like to package materials.
Still another property of paper which makes it undesirable for certain applications is its ready absorption of oils and greases which makes it unsuitable for use in packaging products that contain such oil and greases.
When paper is to be coated with fillers to provide a high quality printing surface, the paper must frequently be smoothed and its porosity reduced so as to minimize the amount of coating required.
In order to avoid some of the above-noted problems, plastic films have been used recently in place of paper in traditional paper applications such as in grocery bags. Polyethylene, for example, is the most common thermoplastic resin used to prepare such plastic films. Typical plastic films have many desirable properties that have made them especially useful in selected applications, not the least of which is their water-resistance. Plastic films can in many instances be soaked directly in water without adversely affecting their strength properties. Moreover, plastic films typically exhibit excellent vapor barrier properties so as to prevent the transmission of water vapor or air. Plastic films also have excellent resistance to oils and greases. Plastic films are also smooth and non-porous.
However, plastic films have a number of distinct disadvantages which limit their use. Plastic films are extremely difficult to print, for example, because of their excellent resistance to water and oils. Conventional oil-based or water-based inks cannot be used to print on plastic film because of their poor adherence thereto. Rather, the inks that are used to print plastic films must contain relatively expensive binders and/or solvents to cause such inks to adhere to the surface of the plastic film. Notwithstanding the use of such relatively expensive binders and solvents, the print quality is often quite poor when compared to the high resolution of printing patterns that is obtainable on paper. Plastic films are also relatively weak as compared to paper on an equivalent weight and thickness basis when the paper is in a dry state. Moreover, plastic films do not have the desirable stiffness of paper nor do they have the desired dead fold characteristics of paper. On a cost-performance basis, finally, plastic film is considerably more expensive than paper.
In order to obtain the desired properties of paper as well as the desired properties of plastic without the undesirable properties of each, it has been suggested to form a plastic layer on the surface of paper and thereby improve the properties of the underlying paper layer. These attempts were not only successful to the extent of improving the moisture resistance of the underlying paper layer, but also to the extent of improving certain other related properties such as vapor barrier properties and grease and oil resistance. The paper layer contributed the required stiffness, dead fold characteristics and overall strength properties to the composite paper which were not obtainable with the plastic film by itself, but the plastic film which protected the underlying paper layer prevented the effective printing of the composite paper.
Therefore, it was recognized in the art that it would be highly desirable to have a composite plastic-paper product which would have all of the desired properties of paper and all of the desired properties of plastic, but which would also be readily printable in manners similar to conventional paper.